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Date: | Thu, 7 Nov 1996 08:25:04 -0500 |
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Jim..
1) We do not allow "Negative Etchback" on our designs. That said, we
have allowed it, as a wavier to the drawing, in some hardware such as test sets or extendard test cards on a "need" basis. To rebuild
the product would have a schedule impact, so we would accept these
"as is", with the supplier notified that he/she would eat any other
non-conforming product.
2) Like you, we are designing/building product to Mil-Std-275 and
we have used the standard Mil formula for end product requirements.
This is what we have been using, for a long time, on our 12-14 layer
polyimide designs.
SMT designs: Min Land Area = A + 2B + C
INTERNAL Worst Case Conditions :
.018 Plated Finished Hole to .040 Land Area
A = .024 max drilled hole
B = .002 Min Annular Ring Requirement
C = .008 Std Fab Allowance
.024 + .004 + .008 = .036 adjusted to .040 pad
(really have .004 more
to play with)
Of course, the key to a real tight max drill size, or as we say--
squeezing the banana, is supplier dependent, SO TALK TO YOUR SUPPLIER!!
3) I am not a board fabricator, but I don't think drill size is related to
etchback... Isn't that process related??
Hope that helps somewhat.
regards,
Bob Vanech
Norden Systems
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